The presently claimed invention relates to a method of producing metal composite materials such as cemented carbide.
Cemented carbide and titanium-based carbonitride alloys (the latter often referred to as cermets) are formed of hard constituents based on carbides, nitrides and/or carbonitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a binder phase essentially based on Co and/or Ni and/or Fe. These alloys are made by powder metallurgical methods comprising milling a powder mixture containing powders forming the hard constituents and binder phase, pressing and sintering.
The milling operation is an intensive milling in mills of different sizes with the aid of milling bodies. The milling time is of the order of several hours up to days. The lengthy milling time is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is further believed that the intensive milling creates a reactivity of the mixture which further promotes the formation of a dense structure.
GB 346,473 discloses a method of making cemented carbide bodies. Instead of milling, the hard constituent grains are coated with a binder phase metal by an electrolytic method, pressed and sintered to a dense structure. This and other similar methods are, however, not suited for cemented carbide production in a large industrial scale and milling is almost exclusively used within the cemented carbide industry today. However, milling has its disadvantages. During the long milling time, the milling bodies wear and contaminate the milled mixture for which contamination, compensation has to be made. The milling bodies can also break during milling and remain in the structure of the sintered bodies. Furthermore, even after an extended milling, a random rather than an ideal homogeneous mixture may be obtained. In order to ensure an even distribution of the binder phase in the sintered structure, sintering has to be performed at a higher temperature than necessary.
Thus, the properties of the sintered metal composite materials containing two or more components depend to a great extent on how well the starting materials are mixed. An ideal mixture of particles of two or more kinds of materials, especially if one of the components occurs as a minor constituent (which is the case for the binder phase in ordinary metal composite materials), is difficult to obtain. In practice, after extended mixing, a random rather than an ideal homogeneous mixture is obtained.
In order to obtain an ordered mixing of the components in the latter case, the minor component can be introduced as a coating. The coating can be achieved by the use of various chemical techniques. In general, it is required that some type of interaction between the coated component and the coating is present, i.e., adsorption, chemisorption, surface tension or any type of adhesion.
U.S. Pat. No. 5,505,902 discloses a method of applying a coating to hard constituent grains. According to the method of said invention, one or more metal salts of at least one iron group metal containing organic groups are dissolved and complex bound in at least one polar solvent with at least one complex former comprising functional groups in the form of OH or NR.sub.3, (R.dbd.H or alkyl). The hard constituent powder and a soluble carbon source are added to the solution. The solvent is evaporated and remaining powder is heat treated in inert and/or reducing atmosphere.
U.S. Pat. No. 5,529,804 discloses another method of applying a coating to hard constituent grains. According to the method of that application, hard constituent powders are coated with cobalt and/or nickel metal in solution by reducing the metals from a suitable salt with a polyol while keeping the powder in suspension. The polyol functions both as a solvent and as a reducing agent at the same time and is present in an amount of at least &gt;5 times more moles polyol than moles metal. There is obtained an even distribution of the cobalt and/or nickel over the surface of the hard constituent powder without the formation of islands of pure metal.